The present invention relates to electric storage batteries or cells and, in particular, to an improved material and method effective in attenuating an explosion of combustible gases which accummulate in the head space of electric storage batteries.
As is well-known in the art, most types of electric storage batteries generate combustible gases during operation, which gases are either vented from the battery container into the atmosphere or are recombined within the battery in secondary reactions with the active materials. However, even in battery constructions which are intended to provide for the internal recombination of combustible gases, there are certain circumstances, such as inadvertent or abusive overcharge, where the recombination mechanism is ineffective and significant volumes of combustible gases will be generated.
It is also well-known that the combustible gases within the head space of a battery may be accidentally ignited and result in explosion of the battery. The damage and injury resulting from such explosions are well documented. Thus, for many years, effective and reliable means have been sought for preventing or minimizing explosions in batteries and the hazardous effects thereof.
The ignition of combustible gases within the head space of a battery can be caused by either an internal or external ignition source. Combustible gases which are generated within a battery, if not effectively recombined, will eventually create a high enough internal pressure so they must be vented to the atmosphere. The venting is typically accomplished through the use of a simple open vent slot or a one-way relief valve, sometimes referred to as a "burp" valve. During venting of combustible gases an external source of ignition, such as a flame or spark near the battery vent, can result in an ignition which will propagate back into the battery container and result in an explosion. However, improvements in relief valve construction and the development of flame arrestors which are used in conjunction with vents have decreased considerably the incidence of battery explosions caused by an external ignition source, provided such protective devices have not been removed or disabled, or the integrity of the container or cover otherwise breached.
However, should an external source of ignition breach one of the protective devices or should an ignition occur within the container, the combustible gases in the head space may explode. The concentration of gases, typically a mixture of hydrogen and oxygen, and the relatively large volume of the head space can result in an explosion which will shatter the container, cover or other components. In addition, the explosion will also often carry with it the liquid acid or other hazardous electrolyte from within the container.
Thus, it is not surprising that materials and methods for suppressing or minimizing the effects of explosions within batteries have been long sought. It is, of course, axiomatic that elimination of the open head space or substantially filling the head space with a solid material would virtually eliminate the possibility of an explosion simply because the presence of combustible gases would be eliminated. However, neither alternative is acceptable. An open head space is necessary in virtually all secondary storage batteries. First of all, the head space accommodates certain battery components, such as plate straps, intercell connectors, or terminals. In addition, in batteries which utilize free liquid electrolyte, sometimes referred to as "flooded" systems, open head space is necessary to accommodate variations in the level of the electrolyte as the battery is cycled or to provide space for acid movement under extreme conditions of use, such as abusive overcharge. Also, the head space accommodates movement of the electrolyte level as the battery is tilted in service, such as the ability to operate an automobile on an incline without loss of electrolyte.
For many years, it has been known to fill the head space in a battery or cell, either partially or totally, with a porous material to inhibit the explosion of gases within the head space and quench any flame which may be formed, while still allowing the movement of gases and electrolyte through the material. For example, U.S. Pat. No. 2,341,382 discloses partially filling the head space with a loosely packed material, such as crushed stone or glass, diatomaceous earth, or glass wool. The disclosure in that patent suggests that the loosely packed filler material will not prevent the explosion of gases entirely, but by dividing the head space chamber into many minute interconnected cells, a rapid total combustion of the gases is prevented and, instead, a series of weak and inconsequential minor explosions will occur until the flame is quenched. It is believed that the general theory set forth in that patent, sometimes called the "chain termination" theory, is essentially correct and valid for a large variety of porous filler materials. However, notwithstanding the soundness of the theory and the development in the ensuing years of many improved porous materials, particularly plastics, there has been no large scale or general implementation of the technology. Thus, there still exists in the battery industry today a serious need for a material and method of utilizing it which will effectively attenuate hazardous explosions, but will otherwise not be detrimental to safe and efficient operation of the battery.
There are a number of factors which are believed to have generally inhibited or prevented the practical and useful application of explosion suppression or attenuation technology in batteries. Broadly, these factors include the creation of other hazards and detrimental effects on battery performance. As the head space of a battery is filled with a porous material, there will be a decrease in the actual remaining void volume in the head space inversely proportional to the porosity or effective void volume of the filler material. In other words, the more solids present in the filler material, the greater will be the reduction in the total head space volume filled with such material. As indicated above and particularly in flooded batteries, the loss of actual open head space volume will lessen the space available for electrolyte movement or electrolyte level variations.
It is known that high rate charging or excessive over-charge can result in vigorous gassing in many types of batteries. If the gas bubbles formed in the electrolyte cannot find ready and fairly direct channels to the battery vent openings, electrolyte may be upwardly displaced and overflow through the battery vents. This condition is known as electrolyte "pumping" and the damaging and hazardous effects of a corrosive electrolyte flowing out of a battery are obvious.
Electrolyte pumping can also occur even where the head space of the battery is filled with a very highly porous material, i.e. a material having a high void volume. For example, an open cell foam material may have a void volume as high as 97 to 98% and, if placed in the head space of a battery, will only occupy about 2 or 3% of the total volume thereof. Nevertheless, in a flooded battery, such a material may readily retain electrolyte and not allow it to drain back into the battery by gravity. Electrolyte so retained in a porous filler material will be readily pumped from the battery under the conditions of vigorous gassing, described above.
In addition, if a relatively large volume of electrolyte is drawn from the cells through wicking by a porous material in the head space or if the porous material otherwise retains the electrolyte with which it comes into contact, insufficient electrolyte may remain in the cells for proper electrochemical reaction and operation of the battery. Also, any material to be used as an attenuation material in batteries must possess certain other critically necessary physical properties. Such materials must have adequate resilience to retain their shape and to readily fill sometimes irregular shape of battery head space. The material must also be thermally and chemically stable in the operating environment within the battery. To provide adequate safety, any attenuation material must be able to survive repeated ignitions without melting or sintering. A material capable of effectively operating only once, but being destroyed in the process, would not be satisfactory. The material cannot, of course, dissolve in or otherwise react with the liquid electrolyte.
A number of porous plastic materials have been used in fuel tanks or similar containers as a means for reducing the explosion hazards. Both fibrous and cellular plastics of various kinds are disclosed in the art. U.S. Pat. No. 3,561,639 discloses the use of a single block of open cell polyurethane foam to fill the interior of a fuel tank. The described material has a reticulated or fully open pore structure, a pore size ranging from 10 to 100 pores per linear inch (ppi), and a void volume of 97%. The fully reticulated structure is described as important to keep flame propagation from reaching the velocity necessary for explosion and to provide a high degree of permeability for the liquid fuel. A material used today for explosion safety in jet aircraft fuel tanks is an ether-base polyurethane foam having a pore size of 20 ppi which is produced by Scott Foam and sold under the name "Protectair".
Bulked fibrous plastic materials of many types have also been proposed for use as a means of arresting flames and reducing explosion hazards in fuel tanks. The filamentary plastic materials proposed for such use include polyolefins, nylon, dacron, polyesters, acrylics, and polyurethanes, as well as others. The materials are typically bulked or textured to provide high porosity and void volume by any of many well-known methods such as twisting, looping, crimping, needle punching and so forth. Examples of various types of such materials are described in U.S. Pat. Nos. 3,650,431, 4,141,460, and 4,154,357.
Notwithstanding the broad use of the foregoing porous plastic materials to suppress explosions in fuel tanks, we are unaware of any effective use of these materials in storage batteries and, in particular, as an explosion attenuation material in the open head space of such batteries. As a result of extensive testing, we have found as a general matter that the materials which perform most effectively to attenuate an explosion and quench the flame resulting from the ignition of combustible gases, do not perform well in other aspects of battery operation. As previously indicated, the violence of an explosion (in terms of the peak pressure developed within the open head space of a battery) can be reduced by substantially filling the head space with a porous material. Small spaces not effectively filled with the porous material and within which combustible gases are ignited will result in minor pressure perturbations. Certain porous materials will attenuate the violence of the explosion and eventually quench the flame. We have found that the pressure developed during an explosion is reduced as the pore size of the attenuation material is decreased. Unfortunately, as the pore size of the material decreases, the adverse effects of the material on battery performance increase. The smaller the pore size of the material, the greater the propensity of the material to wick up electrolyte or to retain within the pores electrolyte with which it is wetted. Electrolyte which is retained in the pores and cannot drain back into the cell can result in two serious problems, as previously mentioned. First, electrolyte retained in the porous material is not readily available for electrochemical reaction and may thus result in diminished electrical performance. Retained electrolyte will also inhibit the flow of gases generated within the battery and, in certain circumstances of operation, result in electrolyte being pumped out of the battery through the vent openings.